Biblio5
.pdfxxii PREFACE
subsidiary organizations, CCITT and CCIR, changed their names to ITU Telecommunication Standardization Sector and the ITU Radio Communications Sector, respectively. Reference publications issued prior to January 1993 carry the older title: CCITT and CCIR. Standards issued after that date carry ITU-T for Telecommunication Sector publications and ITU-R for the Radio Communications Sector documents.
ACKNOWLEDGMENTS
Some authors are fortunate to have a cadre of friends who pitch in to help and advise during the preparation of a book. I am one of these privileged people. These friends have stood by me since the publication of my first technical text. In this group are John Lawlor, principal, John Lawlor and Associates of Sharon, MA; Dr. Ron Brown, independent consultant, Melrose, MA; Bill Ostaski, an expert on Internet matters who is based in Beverly Farms, MA; Marshall Cross, president, Megawave Corp., Boylston, MA; and Jerry Brilliant, independent consultant based in Fairfax, VA.
I am grateful to my friends at Motorola in Chandler, AZ, where I learned about mentoring young engineers. In that large group, four names immediately come to mind: Dr. Ernie Woodward, Doug White, Dr. Ali Elahi, and Ken Peterson—all of the Celestri program.
Then there is Milt Crane, an independent consultant in Phoenix, AZ, who is active in local IEEE affairs. Dan Danbeck, program director with Engineering Professional Development, University of Wisconsin–Madison, who provided constructive comments on the book’s outline. Ted Myers, of Ameritech Cellular, made helpful suggestions on content. John Bellamy, independent consultant and Prof. John Proakis, series editor and well-known author in his own right, reviewed the outline and gave constructive comments to shorten the book to some reasonable length.
I shall always be indebted to Dr. Don Schilling, professor emeritus, City College of New York and great proponent of CDMA in the PCS and cellular environment. Also, my son, Bob Freeman, major accounts manager for Hispanic America, Axis Communications, for suggestions on book promotion. Bob broke into this business about five years ago. Also, my thanks to Dr. Ted Woo of SCTE for help on CATV; to Fran Drake, program director, University of Wisconsin–Madison, who gave me this book idea in the first place; and Dr. Bob Egri, principal investigator at MaCom Lowell (MA) for suggestions on the radio frequency side.
ROGER L. FREEMAN
Scottsdale, Arizona
November, 1998
Fundamentals of
Telecommunications
Fundamentals of Telecommunications. Roger L. Freeman
Copyright 1999 Roger L. Freeman
Published by John Wiley & Sons, Inc.
ISBNs: 0-471-29699-6 (Hardback); 0-471-22416-2 (Electronic)
1
INTRODUCTORY CONCEPTS
1.1 WHAT IS TELECOMMUNICATION?
Many people call telecommunication the world’s most lucrative industry. If we add cellular and PCS users,1 there are about 1800 million subscribers to telecommunication services world wide (1999). Annual expenditures on telecommunications may reach 900,000 million dollars in the year 2000.2
Prior to divestiture, the Bell System was the largest commercial company in the United States even though it could not be found on the Fortune 500 listing of the largest companies. It had the biggest fleet of vehicles, the most employees, and the greatest income. Every retiree with any sense held the safe and dependable Bell stock. In 1982, Western Electric Co., the Bell System manufacturing arm, was number seven on the Fortune 500. However, if one checked the Fortune 100 Utilities, the Bell System was up on the top. Transferring this information to the Fortune 500, again put Bell System as the leader on the list.
We know telecommunication is big business; but what is it? Webster’s (Ref. 1) calls it communications at a distance. The IEEE dictionary (Ref. 2) defines telecommunications as “the transmission of signals over long distance, such as by telegraph, radio or television.” Another term we often hear is electrical communication. This is a descriptive term, but of somewhat broader scope.
Some take the view that telecommunication deals only with voice telephony, and the typical provider of this service is the local telephone company. We hold with a wider interpretation. Telecommunication encompasses the electrical communication at a distance of voice, data, and image information (e.g., TV and facsimile). These media, therefore, will be major topics of this book. The word media (medium, singular) also is used to describe what is transporting telecommunication signals. This is termed transmission media. There are four basic types of medium: (1) wire-pair, (2) coaxial cable, (3) fiber optics, and (4) radio.
1.2 TELECOMMUNICATION WILL TOUCH EVERYBODY
In industrialized nations, the telephone is accepted as a way of life. The telephone is connected to the public switched telecommunications network (PSTN) for local, national,
1PCS, personal communication services, is a cellular-radiolike service covering a smaller operational area. 2We refrain from using billion because it is ambiguous. Its value differs, depending on where you come from.
1
2 INTRODUCTORY CONCEPTS
and international voice communications. These same telephone connections may also carry data and image information (e.g., television).
The personal computer (PC) is beginning to take on a similar role as the telephone, that of being ubiquitous. Of course, as we know, the two are becoming married. In most situations, the PC uses telephone connectivity to obtain internet and e-mail services. Radio adjuncts to the telephone, typically cellular and PCS, are beginning to offer similar services such as data communications (including internet) and facsimile (fax), as well as voice. The popular press calls these adjuncts wireless. Can we consider wireless in opposition to being wired?
Count the number of devices one has at home that carry out some kind of controlling or alerting function. They also carry out a personal communication service. Among these devices are television remote controls, garage-door openers, VCR and remote radio and CD player controllers, certain types of home security systems, pagers, and cordless telephones. We even take cellular radios for granted.
In some countries, a potential subscriber has to wait months or years for a telephone. Cellular radio, in many cases, provides a way around the problem, where equivalent telephone service can be established in an hour—just enough time to buy a cellular radio in the local store and sign a contract for service.
The PSTN has ever-increasing data communications traffic, where the network is used as a conduit for data. PSTN circuits may be leased or used in a dial-up mode for data connections. Of course, the Internet has given added stimulus to data circuit usage of the PSTN. The PSTN sees facsimile as just another data circuit, usually in the dial-up mode. Conference television traffic adds still another flavor to PSTN traffic and is also a major growth segment.
There is a growing trend for users to bypass the PSTN partially or completely. The use of satellite links in certain situations is one method for PSTN bypass. Another is to lease capacity from some other provider. Other provider could be a power company with excess capacity on its microwave or fiber optic system. There are other examples, such as a railroad with extensive rights-of-way that are used by a fiber-optic network.
Another possibility is to build a private network using any one or a combination of fiber optics, line-of-sight-microwave, and satellite communications. Some private networks take on the appearance of a mini-PSTN.
1.3 INTRODUCTORY TOPICS IN TELECOMMUNICATIONS
An overall telecommunications network (i.e., the PSTN) consists of local networks interconnected by a long-distance network. The concept is illustrated in Figure 1.1. This is the PSTN, which is open to public correspondence. It is usually regulated by a government authority or may be a government monopoly, although there is a notable trend toward privatization. In the United States the PSTN has been a commercial enterprise since its inception.
1.3.1 End-Users, Nodes, and Connectivities
End-users, as the term tells us, provide the inputs to the network and are recipients of network outputs. The end-user employs what is called an I/ O, standing for input/ output (device). An I/ O may be a PC, computer, telephone instrument, cellular/ PCS telephone or combined device, facsimile, or conference TV equipment. It may also be some type
1.3 INTRODUCTORY TOPICS IN TELECOMMUNICATIONS |
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Long Distance Network
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Figure 1.1 The PSTN consists of local networks interconnected by a long-distance network.
of machine that provides a stimulus to a coder or receives stimulus from a decoder in, say, some sort of SCADA system.3
End-users usually connect to nodes. We will call a node a point or junction in a transmission system where lines and trunks meet. A node usually carries out a switching function. In the case of the local area network (LAN), we are stretching the definition. In this case a network interface unit is used, through which one or more end-users may be connected.
A connectivity connects an end-user to a node, and from there possibly through other nodes to some final end-user destination with which the initiating end-user wants to communicate. Figure 1.2 illustrates this concept.
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Figure 1.2 Illustrating the functions of end-users, nodes, and connectivity.
3SCADA stands for supervisory control and data acquisition.
4 INTRODUCTORY CONCEPTS
The IEEE (Ref. 2) defines a connection as “an association of channels, switching systems, and other functional units set up to provide means for a transfer of information between two or more points in a telecommunications network.” There would seem to be two interpretations of this definition. First, the equipment, both switching and transmission facilities, is available to set up a path from, say, point A to point B. Assume A and B to be user end-points. The second interpretation would be that not only are the circuits available, but they are also connected and ready to pass information or are in the information-passing mode.
At this juncture, the end-users are assumed to be telephone users, and the path that is set up is a speech path (it could, of course, be a data or video path). There are three sequential stages to a telephone call:
1. Call setup;
2. Information exchange; and
3. Call take down.
Call setup is the stage where a circuit is established and activated. The setup is facilitated by signaling, which is discussed in Chapter 7.4 It is initiated by the calling subscriber (user) going off-hook. This is a term that derives from the telephony of the early 1900s. It means “the action of taking the telephone instrument out of its cradle.” Two little knobs in the cradle pop up, pushed by a spring action, causing an electrical closure. If we turn a light on, we have an electrical closure allowing electrical current to pass. The same thing happens with our telephone set; it now passes current. The current source is a “battery” that resides at the local serving switch. It is connected by the subscriber loop. This is just a pair of copper wires connecting the battery and switch out to the subscriber premises and then to the subscriber instrument. The action of current flow alerts the serving exchange that the subscriber requests service. When the current starts to flow, the exchange returns a dial tone, which is audible in the headset (of the subscriber instrument). The calling subscriber (user) now knows that she/ he may start dialing digits or pushing buttons on the subscriber instrument. Each button is associated with a digit. There are 10 digits, 0 through 9. Figure 1.3 shows a telephone end instrument connected through a subscriber loop to a local serving exchange. It also shows that allimportant battery (battery feed bridge), which provides a source of current for the subscriber loop.
If the called subscriber and the calling subscriber are in the same local area, only
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Figure 1.3 A subscriber set is connected to a telephone exchange by a subscriber loop. Note the battery feed in the telephone serving switch. Distance D is the loop length discussed in Section 5.4.
4Signaling may be defined as the exchange of information specifically concerned with the establishment and control of connections, and the transfer of user-to-user and management information in a circuit-switched (e.g., the PSTN) network.
1.3 INTRODUCTORY TOPICS IN TELECOMMUNICATIONS |
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seven digits need be dialed. These seven digits represent the telephone number of the called subscriber (user). This type of signaling, the dialing of the digits, is called address signaling. The digits actuate control circuits in the local switch, allowing a connectivity to be set up. If the calling and called subscribers reside in the serving area of that local switch, no further action need be taken. A connection is made to the called subscriber line and the switch sends a special ringing signal down that loop to the called subscriber, and her/ his telephone rings, telling her/ him that someone wishes to talk to her/ him on the telephone. This audible ringing is called alerting, another form of signaling. Once the called subscriber goes off-hook (i.e., takes the telephone out of its cradle), there is activated connectivity, and the call enters the information-passing phase, or phase 2 of the telephone call.
When the call is completed, the telephones at each end are returned to their cradles, breaking the circuit of each subscriber loop. This, of course, is analogous to turning off a light; the current stops flowing. Phase 3 of the telephone call begins. It terminates the call, and the connecting circuit in the switch is taken down and freed-up for another user. Both subscriber loops are now idle. If a third user tries to call either subscriber during stages 2 and 3, she/ he is returned a busy-back by the exchange (serving switch). This is the familiar “busy signal,” a tone with a particular cadence. The return of the busy-back is a form of signaling called call-progress signaling.
Suppose now that a subscriber wishes to call another telephone subscriber outside the local serving area of her/ his switch. The call setup will be similar as before, except that at the calling subscriber serving switch the call will be connected to an outgoing trunk. As shown in Figure 1.4, trunks are transmission pathways that interconnect switches. To repeat: subscriber loops connect end-users (subscriber) to a local serving switch; trunks interconnect exchanges or switches.
The IEEE (Ref. 2) defines a trunk as “a transmission path between exchanges or central offices.” The word transmission in the IEEE definition refers to one (or several) transmission media. The medium might be wire-pair cable, fiber optic cable, microwave radio and, stretching the imagination, satellite communications. In the conventional telephone plant, coaxial cable has fallen out of favor as a transmission medium for this application. Of course, in the long-distance plant, satellite communication is
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Figure 1.4 Subscriber loops connect telephone subscribers to their local serving exchange; trunks interconnect exchanges (switches).
6 INTRODUCTORY CONCEPTS
fairly widely employed, particularly for international service. Our preceding reference was for local service.
1.3.2 Telephone Numbering and Routing
Every subscriber in the world is identified by a number, which is geographically tied to a physical location.5 This is the telephone number. The telephone number, as we used it here, is seven digits long. For example:
234 − 5678
The last four digits identify the subscriber line; the first three digits (i.e., 234) identify the serving switch (or exchange).
For a moment, let’s consider theoretical numbering capacity. The subscriber number, those last four digits, has a theoretical numbering capacity of 10,000. The first telephone number issued could be 0000, the second number, if it were assigned in sequence, would be 0001, the third, 0002, and so on. At the point where the numbers ran out, the last number issued would be 9999.
The first three digits of the preceding example contain the exchange code (or central office code). These three digits identify the exchange or switch. The theoretical maximum capacity is 1000. If again we assign numbers in sequence, the first exchange would have 001, the next 002, then 003, and finally 999. However, particularly in the case of the exchange code, there are blocked numbers. Numbers starting with 0 may not be desirable in North America because 0 is used to dial the operator.
The numbering system for North America (United States, Canada, and Caribbean islands) is governed by the NANP or North American Numbering Plan. It states that central office codes (exchange codes) are in the form NXX where N can be any number from 2 through 9 and X can be any number from 0 through 9. Numbers starting with 0 or 1 are blocked numbers. This cuts the total exchange code capacity to 800 numbers. Inside these 800 numbers there are five blocked numbers such as 555 for directory assistance and 958/ 959 for local plant test.
When long-distance service becomes involved, we must turn to using still an additional three digits. Colloquially we call these area codes. In the official North American terminology used in the NANP is NPA for numbering plan area, and we call these area codes NPA codes. We try to assure that both exchange codes and NPA codes do not cross political/ administrative boundaries. What is meant here are state, city, and county boundaries. We have seen exceptions to the county/ city rule, but not to the state. For example, the exchange code 443 (in the 508 area code, middle Massachusetts) is exclusively for the use of the town of Sudbury, Massachusetts. Bordering towns, such as Framingham, will not use that number. Of course, that exchange code number is meant for Sudbury’s singular central office (local serving switch).
There is similar thinking for NPAs (area codes). In this case it is that these area codes may not cross state boundaries. For instance, 212 is for Manhattan and may not be used for northern New Jersey.
Return now to our example telephone call. Here the calling party wishes to speak
5This will change. At least in North America, we expect to have telephone number portability. Thus, whenever one moves to a new location, she/ he takes her/ his telephone number with them. Will we see a day when telephone numbers are issued at birth, much like social security numbers?
1.3 INTRODUCTORY TOPICS IN TELECOMMUNICATIONS |
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447 |
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Figure 1.5 Example connectivity subscriber-to-subscriber through two adjacent exchanges.
to a called party that is served by a different exchange (central office).6 We will assign the digits 234 for the calling party’s serving exchange; for the called party’s serving exchange we assign the digits 447. This connectivity is shown graphically in Figure 1.5. We described the functions required for the calling party to reach her/ his exchange. This is the 234 exchange. It examines the dialed digits of the called subscriber, 447–8765. To route the call, the exchange will only work upon the first three digits. It accesses its local look-up table for the routing to the 447 exchange and takes action upon that information. An appropriate vacant trunk is selected for this route and the signaling for the call advances to the 447 exchange. Here this exchange identifies the dialed number as its own and connects it to the correct subscriber loop, namely, the one matching the 8765 number. Ringing current is applied to the loop to alert the called subscriber. The called subscriber takes her/ his telephone off-hook and conversation can begin. Phases 2 and 3 of this telephone call are similar to our previous description.
1.3.3 Use of Tandem Switches in a Local Area Connectivity
Routing through a tandem switch is an important economic expedient for a telephone company or administration. We could call a tandem switch a traffic concentrator. Up to now we have discussed direct trunk circuits. To employ a direct trunk circuit, there must be sufficient traffic to justify such a circuit. One reference (Ref. 3) suggests a break point of 20 erlangs.7 For a connectivity with traffic intensity under 20 erlangs for the busy hour (BH), the traffic should be routed through a tandem (exchange). For traffic intensities over that value, establish a direct route. Direct route and tandem connectivities are illustrated in Figure 1.6.
1.3.4 Busy Hour and Grade of Service
The PSTN is very inefficient. This inefficiency stems from the number of circuits and the revenue received per circuit. The PSTN would approach 100% efficiency if all the circuits were used all the time. The fact is that the PSTN approaches total capacity utilization for only several hours during the working day. After 10 P.M. and before 7 A.M. capacity utilization may be 2% or 3%.
The network is dimensioned (sized) to meet the period of maximum usage demand.
6 The term office or central office is commonly used in North America for a switch or an exchange. The terms switch, office, and exchange are synonymous.
7The erlang is a unit of traffic intensity. One erlang represents one hour of line (circuit) occupancy.
8 INTRODUCTORY CONCEPTS
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Figure 1.6 Direct route and tandem connectivities. |
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This period is called the busy hour (BH). There are two periods where traffic demand on the PSTN is maximum–one in the morning and one in the afternoon. This is illustrated in Figure 1.7.
Note the two traffic peaks in Figure 1.7. These are caused by business subscribers. If the residential and business curves were combined, the peaks would be much sharper. Also note that the morning peak is somewhat more intense than the afternoon busy hour. In North America (i.e., north of the Rio Grande river), the busy hour BH is between 9 : 30 A.M. and 10 : 30 A.M. Because it is more intense than the afternoon high-traffic period, it is called the BH. There are at least four distinct definitions of the busy hour. We quote only one: “That uninterrupted period of 60 minutes during the day when the traffic offered is maximum.” Other definitions may be found in (Ref. 4).
BH traffic intensities are used to dimension the number of trunks required on a connectivity as well as the size of (a) switch(es) involved. Now a PSTN company (administration) can improve its revenue versus expenditures by cutting back on the number
Figure 1.7 The busy hour.